The Fertilizer Tree - Red Alder

Did you know there is a tree in our local forests that adds fertilizer to the soil? It is the red alder (Alnus rubra), and it does the same thing for the forest as alfalfa does for the farm. It makes   nitrate fertilizer. To be more accurate the microscopic partners living in the alder roots make the fertilizer. Alder roots have clusters of little round nodules on them, and in these nodules there is a special kind of bacteria. It is called Frankia alni, an actinomycete. The actinomycete bacteria are medicinally very important, as this group of organisms produce two thirds of our antibiotics. Legumes such as alfalfa and peas have similar bacteria-containing nodules, although they contain a different group of bacteria. The bacteria take nitrogen out of the air and combine it with oxygen to make nitrate. The nitrate is then taken by the tree to build its proteins. In return the alder roots provide a home for the Frankia. When alder leaves fall in the autumn their rich stores of nitrogen go into the soil as fertilizer. Over the years this rich humus accumulates and transfers its nutrients to other plants.

Have you also noticed that alder leaves are still green when they fall in the autumn. Other deciduous leaves turn yellow, red, or purple as the summer draws to a close but not those of alder. There is a good reason for this. Other trees take some of the old nutrients from the dying leaves and store them in the outer part of the trunk and branches, or in next year’s buds. This provides an extra spurt of nutrition to prime the first growth early in the spring, before photosynthesis gets under way. The dying leaves stay on the trees for a while until this food source is re-claimed. Alder leaves drop from the branches while there is still lots of chlorophyll in them. They may turn yellow, but only after they have been shed. Alders do not need to store their valuables. The microscopic allies supply them with all the fertilizer they require. 

Alder prepares the way for the coniferous trees which come later. It’s seedlings are the first to germinate after a fire, logging, or on a bare river bank. It grows rapidly, and is short-lived for a tree, about 70 or 80 years. An abandoned logging road can be home for thousands of young alders. In order to germinate they require sunlight and bare gravely or sandy soil. They do not grow in shade or in humus. If you walk through an alder woodland you will not see baby alder trees, because they cannot grow in the shade or in the rich soil that the mature alders have created. Instead the seed, which possesses a small wing, must be transported by the wind to another bare disturbed area. The young trees growing within the alder forest are conifers such as redcedar and western hemlock. These trees live up to ten times as long as the alders which preceded them. In an old hemlock or cedar forest there is no evidence remaining of the alder forest which came first, but it was the nutrients from alders that nurtured the big conifers when they were babies. 

Take a close look at the bark of an old alder. You will see two obvious things, one of which belongs to the alder, and another that many people think belongs to the alder but doesn’t. Alder bark is essentially smooth, except for tiny, pale colored bumps on its surface. These bumps, called lenticels, are softer than the bark itself, and are the breathing holes of the tree. Bark is impervious and one of its functions is to prevent disease from entering the tree. This is why it is not a good idea to chop pieces out of trees. The air is a whole ecosystem of fungus spores looking for a wounded tree where they can build a new home. Bark is also impervious to air and water, and many trees and shrubs produce lenticels which are porous spots where oxygen can diffuse into the tissues below. Lenticels are most easily seen on birch and cherry trees. The horizontal lines on the trunks of these trees are big lenticels, where the trunks breathe.

The features which look like part of the bark, but are not, are the pale splotches which many of the alders sport. These are separate living organisms that use the bark as a surface on which to grow. They are primitive lichens, and there are several different kinds growing on alder trunks. If you look closely at the splotches you will often see small details on them, and the details often differ from one lichen to another. The small markings on them are the reproductive bodies of the lichens, and if they look different from one splotch to another splotch it is because the splotches are different kinds of lichens. If there are no markings on the crusts, the different kinds look essentially the same.

If there are black squiggles, like cuneiform writing, on the surface you have a script lichen (Graphis scripta). Both the scientific and common names refer to the fact that the black markings look like writing. The lichen reproduces by producing spores in the dark lines. Rather, half the lichen reproduces by producing spores. A lichen is a composite organism - two in one. Most of it is composed of fungus. Imbedded deep inside the fungus coating are single cells of a microscopic plant. Like all plants this one uses photosynthesis to make sugar, and the fungus makes its living and builds its structure by stealing sugar from the plant. The spores float away on the wind, with an almost non-existent possibility they will land on the right plant cells to regenerate a lichen. All this, of course, occurs on a microscopic level, and nobody has ever seen the process taking place. Our knowledge of what takes place is based on circumstantial evidence.

Script lichen (Graphis scripta) on Red Alder

Photo by Kelly Sekhon

Other common lichens which form these bark patches are the rim-lichens (Lecanora spp.), that have tiny disc-shaped reproductive structures on the surface, and the bitter wart lichen (Pertusaria amara) with tiny, dusty warts. 

During the winter when food supplies are hard to come by, and cold dark days are the norm the alder produces another kind of sustenance. It is on the bare branches of the alders where the songbirds congregate. The little cones are densely packed with tiny seeds, which supply the birds with food over this time of famine. Cracks in the bark also conceal small insects, providing the birds with a little extra protein.

As the winter draws to a close an alder forest announces its presence in a subtle, but colorful way. Alder buds are a pinkish color, as are the young male catkins. These are elongated clusters of densely packed flowers, but look different from what we generally consider flowers because they are wind pollinated. Therefore, they do not have showy petals to attract bees or other insects. As spring approaches the catkins grow in the crowns of the trees. At this time there are no leaves on these trees, and the branches are readily visible. Alders usually grow in pure stands, and the masses of young catkins, combined with young buds give a pink glow to the slopes of many a hillside, announcing the approach of spring.

At one time alders were considered weed trees, as the wood is not as valuable as that of some of the coniferous trees. Foresters now have a better understanding of alder’s role in the forest, and forest practices have also changed with changes in knowledge and economics. Therefore, alder trees are viewed in a more favorable light than in previous decades. Next you go hiking in a deciduous woodland take a closer look at this tree which is doing so much for the health of a future forest which we will never see.